Goal:
    x : V.t  y : V.t  p : VSet.t  a : V.t
  ============================
   ((x = a \/ y = a \/ VSet.In a p -> y = a \/ x = a \/ VSet.In a p) /\
    (y = a \/ x = a \/ VSet.In a p -> x = a \/ y = a \/ VSet.In a p))


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro